Traditionally, a new project for a gas pipeline system starts in a technical economical evaluation phase which considers a steady state flow analysis with a lot of guesswork and assumptions, such as market delivery build up, minimum and maximum flow and estimated number of compressor stations. For a reasonable evaluation of the system a load factor is normally adopted for the gas pipeline, which means that the pipeline will be calculated with a constant flow which will be higher than the nominal flow considered to be supplied to the foreseen market. The main difficulty is to define a duty load that would represent the system under analysis since different flow profiles,' operating pressures, diameter and length of the pipeline will affect the behavior of the complete system. As soon as the technical economic evaluation shows an attractive return rate for the project and the companies involved decide on a go ahead position, the design phase takes place. It is in this phase that a transient analysis for the system has proved to be a must as we have experienced in the design of the Bolivia-Brazil Gas Pipeline. It is a 32" diameter and 1127 miles (1813 km) long pipeline from Rio Grande (Santa Cruz de La Sierra - Bolivia) to Campinas (SQo Paulo - Brazil) with 14 compression stations. It is designed for 1.043 Bcf/d (30 MM m3/d) of natural gas. Why do a transient analysis in the design phase? The first reason is related to the large investment that is required for such a system which includes pipeline and compression stations. The system designed must be able to operate in the predicted different scenarios, othewise the transportation company would face penalties for not delivering the contractual volumes of gas or make additional capital investment on the system, dramatically affecting the cash flow of the project. A second reason is that in this phase normally we face the inclusion of new deliveries with different profiles such as thermal generation that may interrupt the gas consumption to zero for a certain time on a weekly basis, as will normally happen in Brazil. These scenarios must be taken into account since they directly affect the schedule of placing compression stations and units into operation depending on the gas build up delivery. Additionally we may use the transient analysis to make a pre-definition of the turbo compressors that best fit our system and also define the best arrangement for the units in the station, whether in a series (few units and bigger machines) or parallel arrangement (more units and smaller machines). In line with this we may do a failure analysis for a single unit or for a complete station and foresee how the system would cope with that, determine its remaining capacity, and also define a much better maintenance procedure, or even detect a necessity of a standby unit in the worst cases.

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